I'll leave the humour with you, MP, you are doing a top job at it!
The world being huge IS the problem 
Drone delivery only works if you have nodal warehouses in the city in which you want to deploy the service. This goes against the little trick Bezos used to make Amazon work in the first place; it introduces additional overhead for a service that is only used by a fraction of all Amazon customers, and can only be applied to a fraction of the inventory Amazon ships and sells.
Again, you are misplacing the usage of your intelligence here. You are really smart but I think you are thinking on the wrong problems (and the wrong ideas, there are certain things you are missing out and I'll get to them in a minute!).
However, I will entertain your questions. I think this "overhead" will pay itself out in the middle term very easily. Let's make a case study. Let's take the really pessimistic case that San Francisco city is really different than the rest of the world, and while the rest of the world stays in StatusQuoLand, SF will try out this tech. Let's also assume only 1% of Amazon's clients in SF will desire this utility.
Let's also assume SF has 1.000.000 clients for Amazon. This is probably on the really low end figure (I'm counting the whole city + suburbia). 1% is ten thousand clients. These clients will be offered service by a small helipad on their huge warehouses near San Francisco and a shifting number of 'copters (built as needed). That's the whole overhead we are talking here. A robot delivers the package to the copter, the copter picks it up and delivers it from a small helipad. The rest of the service continues as it has ever continued in the past.
The thing I said you are missing here is the 30 minute window and its importance in "impulse buying". If you can really get the buying time down to a 30 minute windowframe, the selling opportunities are really big. Also coming,
renting. You need stuff right now badly. You have your phone, use the amazon app to buy/rent some important "thing", in 30 minutes you get it at your GPS location. This is a whole new market that opens up.
The technicalities may well "fail" on the safe side here, and the more I think about it, the more impressed I am with the idea.
The dangers (drones) are also real. So I think the real discussion is not "if", but "when" and "what then".